Fuel cell system and method of controlling the same

ABSTRACT

In a fuel cell system and its controlling method, the fuel cell system includes a stack  21  including fuel cells  11  each having a polymer electrolyte membrane  13 , and a controller  61 . The controller  61  is responsive to detected outputs of a displacement sensor  27  and a temperature sensor  27  and controls such that, when the polymer electrolyte membrane  13  is discriminated to remain in an excessively dry state, a shut-off valve  37  is applied with a “close” control signal to interrupt the supply of fuel gas to the stack  21  and, concurrently, a shut-off valve  41  is applied with an “open” control signal to allow air to be supplied to the stack  21  while applying a pump control signal to a pump  57  so as to maximize its rotational speed for thereby increasing the flow rate of pure water  59  to be circulated to the humidifier  35  from a pure water tank  55 . Simultaneously, a timer of the controller  61  is operated to begin counting an incremental time. As a result, air is excessively humidified by the humidifier  35  and is supplied to the stack  21  via the shut-off valve  41.

TECHNICAL FIELD

[0001] The present invention relates to a fuel cell system and a methodof controlling the same, and more particularly, to a fuel cell controlsystem of a polymer electrolyte type and a method of controlling a fuelcell of the polymer electrolyte type.

BACKGROUND ART

[0002] In recent years, considerable research and development work hasbeen undertaken to commercially apply a fuel cell of polymer electrolytemembrane type, which has a high power density and which can be operatedat low temperature, as an electric power generation system in a motorvehicle.

[0003] Such a fuel cell of the polymer electrolyte membrane is usuallyconstructed of a polymer electrolyte membrane, an anode joined to oneside of the membrane, and a cathode joined to the other side of themembrane to provide a joined structure, which is sandwiched betweenseparators.

[0004] In a fuel cell system including a plurality of fuel cells ofpolymer electrolyte membrane type as a stack, fuel gas and air areusually humidified by a humidifier for preventing the polymerelectrolyte membrane from being dried such that the polymer electrolytemembrane is kept in a suitably wet state throughout electric powergeneration.

DISCLOSURE OF INVENTION

[0005] In such a fuel cell system, however, a usual practice normallydone in usual operation of the stack has been adapted to start up thestack, even when the fuel cell system is left in a non-use state for along time period and the polymer electrolyte membrane remains in anexcessively dry state.

[0006] For this reason, humidification of the polymer electrolytemembrane to a sufficiently wet state needs a preliminary longeroperating time to some extent, with a resultant difficulty in taking outelectric power output from the stack in a stable fashion or anundesirable system failure owing to rapid voltage drop caused when alarge amount of electric power output is required.

[0007] The present invention has been made in view of theabove-described studies and has an object to provide a fuel cell systemand a method controlling the same which can achieve a substantiallyoptimum start-up control even when a fuel cell system of a polymerelectrolyte type remains in a dry state.

[0008] A fuel cell system of the present invention is provided with: ahumidifier humidifying fuel gas and air; a stack producing electricpower by reacting the fuel gas humidified by the humidifier and the airhumidified by the humidifier, including a plurality of fuel cells andfixed for free movement in a stacked direction thereof, each of theplurality of fuel cells having a polymer electrolyte membrane; adisplacement sensor detecting a displacement value in length of thestack in the stacked direction; a temperature sensor detecting atemperature of the stack; and a controller discriminating whether thepolymer electrolyte membrane is in a dry state in response to thedisplacement value of the stack detected by the displacement sensor andthe temperature of the stack detected by the temperature sensor andcontrolling the humidifier, when the polymer electrolyte membrane isdiscriminated as in a dry state at a start of operation of the stack, tocause the polymer electrolyte membrane to be brought into a wet state.

[0009] In other words, a fuel cell system of the present invention isprovided with: humidifying means humidifying fuel gas and air; a stackproducing electric power by reacting the fuel gas humidified by thehumidifying means and the air humidified by the humidifying means,including a plurality of fuel cells and fixed for free movement in astacked direction thereof, each of the plurality of fuel cells having apolymer electrolyte membrane; displacement detecting means detecting adisplacement value in length of the stack in the stacked direction;temperature detecting means detecting a temperature of the stack; andcontrolling means discriminating whether the polymer electrolytemembrane is in a dry state in response to the displacement value of thestack detected by the displacement detecting means and the temperatureof the stack detected by the temperature detecting means and controllingthe humidifying means, when the polymer electrolyte membrane isdiscriminated as in a dry state at a start of operation of the stack, tocause the polymer electrolyte membrane to be brought into a wet state.

[0010] Besides, a method of controlling a fuel cell system is applied toa system provided with a humidifier humidifying fuel gas and air, and astack producing electric power by reacting the fuel gas humidified bythe humidifier and the air humidified by the humidifier, including aplurality of fuel cells and fixed for free movement in a stackeddirection thereof, each of the plurality of fuel cells having a polymerelectrolyte membrane. The method detects a displacement value in lengthof the stack in the stacked direction and a temperature of the stack,discriminates whether the polymer electrolyte membrane is in a dry statein response to the displacement value of the stack detected by thedisplacement sensor and the temperature of the stack detected by thetemperature sensor, and controls the humidifier, when the polymerelectrolyte membrane is discriminated as in a dry state at a start ofoperation of the stack, to cause the polymer electrolyte membrane to bebrought into a wet state.

[0011] Other and further features, advantages, and benefits of thepresent invention will become more apparent from the followingdescription taken in conjunction with the following drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0012]FIG. 1 is a view for illustrating a cell structure of a polymerelectrolyte membrane type fuel cell to be incorporated in a fuel cellsystem according to a first preferred embodiment of the presentinvention;

[0013]FIG. 2 is a schematic view for illustrating a typical example of afixing structure for a stack constructed of a plurality of fuel cellsaccording to the embodiment;

[0014]FIG. 3 is a view for illustrating a displacement characteristic inlength of the stack in a stacked direction depending on differences in awet state of the stack according to the embodiment;

[0015]FIG. 4 is a block diagram of a starter device for a fuel cellsystem according to the embodiment;

[0016]FIG. 5 is a flow diagram to illustrate the operation of thestarter device of the fuel cell system according to the embodiment;

[0017]FIG. 6 is a block diagram of a starter device of a fuel cellsystem according to a second preferred embodiment of the presentinvention; and

[0018]FIG. 7 is a flow diagram to illustrate the operation of thestarter device of the fuel cell system according to the embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

[0019] To describe the present invention in more detail, preferredembodiments of the present invention will be explained with reference tothe drawings below.

[0020] (First Embodiment)

[0021]FIG. 1 shows a cell structure of a polymer electrolyte type fuelcell for a fuel cell system of a preferred embodiment according to thepresent invention.

[0022] In FIG. 1, the fuel cell 11 is constructed having a polymerelectrolyte 13, an anode electrode 15 located on one side of the polymerelectrolyte membrane 13, a cathode 17 located on the other side of thepolymer electrolyte membrane 13 to form a membrane electrode assembly(MEA), which is sandwiched between separators 19.

[0023]FIG. 2 is a view illustrating a typical example of a fixingstructure of a stack which is comprised of a plurality of fuel cells.

[0024] Since the stack 21 is constructed of a large number of fuel cells11, the stack 21 causes the polymer electrolyte membrane 13 to swell dueto humidifying water, with a resultant expansion and contraction causedin each of the separators 19 in a stacked direction X owing to thermalexpansion.

[0025] For this reason, one distal end of the stack 21 is fixedlyfastened to a vehicle body 25 by means of a stationary fixture 23, andthe other distal end of the stack 21 is connected to the vehicle body 25via a movable fixture 24 so as to prevent vertical movement in adirection Z while allowing expanding and contracting movement in thestacked direction X. Also, a displacement sensor 27 is located on themovable fixture 24 and detects current displacement value relative to anoriginal length of the stack 21 measured at a fabricating stage thereof,producing a displacement detection signal 27 a which is delivered to acontroller 61 in a manner as will be described in detail.

[0026]FIG. 3 is a view for illustrating the displacement characteristicof the stack 21 in conjunction with a varying length in the stackeddirection X due to a difference in a wet state of the stack 21.

[0027] In the event the polymer electrolyte membrane 13 of the stack 21does not contain water immediately when it has been fabricated or whenthe stack 21 has been left in anon-operating state for a long timeperiod to cause the stack 21 to remain in an extremely dry state, thestack 21 remains in a minimum displacement position P1, representing theshortest length of the stack and an extremely dry state, as shown inFIG. 3.

[0028] On the contrary, when the stack 21 is continuously humidified ina suitably wet state and is operating at the maximum power output, thestack 21 encounters swelling and thermal expansion to assume a properwet position P5, equal to the longest length of the stack, as shown inFIG. 3 during electric power generation. Further, when the operation ofthe fuel stack 21 is repeated at suitable periods, the polymerelectrolyte membrane 13 is liable to contain water and, therefore, thestack 21 assumes a normal wet position P3, equal to an intermediatelength closer to the proper wet position P5, as shown in FIG. 3 evenwhen the stack 21 remains in a normal non-operating state.

[0029]FIG. 4 is a block diagram showing a starter device 31 of a fuelcell system of a first preferred embodiment according to the presentinvention. A structure of the starter device 31 of the fuel cell systemS of FIG. 3 will be mainly described below in detail.

[0030] Fuel gas 33 is hydrogen gas or reformed gas delivered from forexample a reformer (not shown) and is supplied to a humidifier 35 whichhumidifies fuel gas 33. Humidified fuel gas is then supplied through ashut-off valve 37 to the stack 21. Also, air 39 is supplied from forexample an air compressor (not shown) and is humidified in thehumidifier 35. Humidified air is then supplied through a shut-off valve41 to the stack 21.

[0031] In the stack 21, humidified fuel gas and humidified air arereacted to produce electric power output, with non-reacted exhaust gas43 being emitted and non-reacted air 45 being also exhausted through acondenser 47 which emits exhaust air 49. Further, a temperature sensor28 is mounted on the stack 21 for detecting the temperature thereof andproducing a temperature detection signal 28 a which is applied to thecontroller 61.

[0032] In the condenser 47, air 45 emitted from the stack 21 is passedthrough plural cooling fins through which coolant 51 is circulated,thereby condensing surplus water contained in air 45 to recover thesame. Recovered pure water 53 is fed to a pure water tank 55.

[0033] The pure water tank 55 serves to capture ions from pure water 53with the use of an ion filter and stores the same therein and,subsequently, pure water 59 is pressurized with a pump 57 and suppliedto the humidifier 35.

[0034] The controller 61 includes a RAM (not shown) which stores controldata, a ROM (not shown) which stores control programs, a CPU (not shown)which controls a system in accordance with the control programs, and atimer (not shown) which counts preset increment times to produce aninterrupting signal INT which is applied to the CPU.

[0035] The controller 61 is responsive to the displacement detectionsignal 27 a delivered from the displacement sensor 27 and thetemperature detection signal 28 a delivered from the temperaturedetection sensor 28 to discriminate whether the polymer electrolytemembrane located in the stack 21 remains in a dry state or is in a wetstate, thereby producing a pump control signal 61 a so as to control theshut-off valves 37 and 41 and the pump 57 such that, in the event of anexcessively dry state, they are operated in a dry-state start sequenceand, in the event of the wet state, they are switched over to and arecontrolled to start operation in a normal start sequence.

[0036] A DC/DC converter 63 is applied with electric power output fromthe stack 21 and functions to achieve a voltage boost conversion orvoltage lowering conversion responsive to a demanded power commandsignal 61 b delivered from the controller 61 for thereby controlling andlimiting the amount of electric power output, to be delivered from thestack 21, which is supplied through the DC/DC converter 63 to a battery(not shown) which serves as a load, and other load units (not shown).

[0037] Now, the operation of the starter device 31 of the fuel cellsystem of the first preferred embodiment is described below in detailwith reference to a general flow diagram shown in FIG. 5. The basicsequence of operations illustrated in the general flow diagram of FIG. 5is stored as the control program in the internal ROM of the controller61.

[0038] At the start-up operation, power is applied to the starter device31 of the fuel cell system, thereby starting up operation of thecontroller 61. At this instant, the controller 61 begins to read out thecontrol program stored in the ROM and to control in a manner describedbelow.

[0039] In a first execution of step S10, the displacement value L in thelength of the stack 21 and the temperature T thereof are watched anddetected at all times by the displacement sensor 27 and the temperaturesensor 28, respectively.

[0040] In conjunction with the displacement value L of the stack 21, thelength of the stack 21, which has been originally measured at thefabrication stage, is preliminarily stored as a reference data in theRAM of the controller 61. Also, when the stack 21 is replaced with newone, it is required to renew this reference data. Further, the RAM ofthe controller 61 preliminarily stores therein parameters covering otherdisplacement values caused by thermal expansion responsive to the stacktemperature T.

[0041] In the next step S20, the wet state of the polymer electrolytemembrane is discriminated in terms of the displacement value L and thetemperature T which are detected by the displacement sensor 27 and thetemperature sensor 28, respectively. A process for discriminating thewet state of the polymer electrolyte membrane will be described indetail below.

[0042] For instance, in a case where a separator is made of for examplecarbon, the instantaneous value is obtained from the original length Ameasured at the fabricating stage, the length B measured when the stackis held in a non-operating state under a suitable wet state (namely,when the stack is held in the non-operating state at a normaltemperature), the length C measured when the stack is operating in anormal state at the maximum power output, and the maximum displacementvalue d, caused by thermal expansion, of the stack, i.e. is expressed bythe following relation:

A+d<B−d  (1)

[0043] Also, the maximum displacement valued of the stack caused bythermal expansion is derived from the coefficient β of linear expansionin the stacked direction X, the temperature T and the length A′ measuredat 0° C. during the fabricating step and is expressed as:

d=β×T×A′  (2)

[0044] With this equation (2), the formula (1) is expressed as:

A+β×T×A′<B−β×T×A′  (3)

[0045] The formula (1) is proved from a reason in that although thecoefficient of thermal expansion of the separator made of carbon is onthe order of 10⁻⁶, the amount of swelling (,i.e. though it depends onthe film thickness and a chemical structure of the polymer electrolytemembrane) of the polymer electrolyte membrane due to wet state is on theorder of 10⁻¹.

[0046] For this reason, the stack encounters thermal expansion which islarger in value at all times when the stack temperature is low under asuitable wet state in the polymer electrolyte membrane, than thatencountered when the stack temperature is high under an extremely drystate in the polymer electrolyte membrane.

[0047] Accordingly, in the event the length L of the stack detected bythe displacement sensor 27 at the start of operation is obtained by:

L<B−d=B−β×T×A′  (4)

[0048] that is, when the length L of the stack is less than theinstantaneous value in which the maximum displacement value d caused bythermal expansion is subtracted from the length B of the stack measuredwhen it is held in the non-operating state under the suitable wet state,it can be found that the polymer electrolyte membrane remains in theexcessively dry state.

[0049] In the second execution of step S20, also, the controller 61discriminates on the basis of the detected values of the displacementsensor 27 and the temperature sensor 28 that the polymer electrolytemembrane remains in the excessively dry state and, in this event, theoperation goes to step S30.

[0050] In step S30, the shut-off valve 37 located between the humidifier35 and the stack 21 is applied with a “close” control signal forinterrupting the supply of fuel gas to the stack 21 and, concurrently,the shut-off valve 41 located between the humidifier 35 and the stack 21is applied with an “open” control signal for allowing air to be suppliedto the stack 21. Simultaneously, the pump 57 is applied with the pumpcontrol signal which allows the rotational speed to be maximized,thereby increasing the flow rate of pure water 59 to be circulated tothe humidifier 35 from the pure water tank 55. At the same time, thetimer in the controller 61 begins to count the incremental time. As aresult, air, which is excessively humidified by the humidifier 35, issupplied to the stack 21 through the shut-off valve 41.

[0051] In the succeeding step S40, the counted incremental time is readout from the timer and the polymer electrolyte membrane of the stack 21is humidified until the humidifying time period reaches a predeterminedtime period Sh.

[0052] When the humidifying time period reaches the predeterminedincremental time Sh, the operation goes to step S50 and, in this step,in addition to supplying the air, the shut-off valve 37 located betweenthe humidifier 35 and the stack 21 is applied with an “open” controlsignal to begin the supply of fuel gas to the stack 21 while applyingthe pump control signal 61 a to the pump 57 such that the flow ratethereof is returned to its normal value to allow the stack 21 to beginpower generation.

[0053] Then in step S60, in the same manner as previously noted in stepS10, the displacement sensor 27 and the temperature sensor 28 detect thedisplacement value L of the length of the stack and the temperaturethereof.

[0054] In the succeeding step S70, in the same manner as previouslynoted in step S20, the controller 61 responds to the detected values ofthe displacement sensor 27 and the temperature sensor 28, respectively,and if it is found that the polymer electrolyte membrane remains in thedry state, the operation returns to step S80.

[0055] In step S80, the amount of electric power output to be extractedfrom the stack 21 is regulated to a limited value below a predeterminedvalue Ph. In particular, the electric power output of the stack 21 isdelivered to the DC/DC converter 63, which converts the voltage upwardor downward in response to the demanded power command signal 61 bapplied from the controller 61 and controls the amount of electric powerto be extracted from the stack 21 to the limited value below thepredetermined value Ph, thereby allowing electric power output from theDC/DC converter 63 to the battery (not shown) and the other load units(not shown). Operation then returns to step S60 and the above discussedprocess is repeated.

[0056] In the execution of step S70, if it is found that the polymerelectrolyte membrane is not held in the dry state and remains in asufficiently wet state, operation goes to step S90 to allow the stack 21to produce electric power output in a normal operating mode. Inparticular, the demanded power command signal 61 b applied from thecontroller 61 to the DC/DC converter 63 is modulated to have a normalpower level by which the DC/DC converter 63 controls for relaxing thelimitation in voltage boost conversion or voltage lowering conversion,thereby allowing normal electric power to be supplied from the DC/DCconverter 63 to the battery (not shown) and the other load units (notshown).

[0057] Thus, it can be found on the basis of the length L of the stackand the temperature T thereof that the polymer electrolyte membranelocated in the stack 21 is sufficiently wet, and the amount of electricpower to be utilized is regulated to the limited value below thepredetermined value Ph until the maximum electric power output isobtained. As a result, it is possible to suppress undesirable situationssuch as a sudden stop caused in a system owing to some reasons such asrapid voltage drop.

[0058] In the execution of step S20, if it is found by the controller 61on the basis of the detected values of the displacement sensor 27 andthe temperature sensor 28, respectively, that the polymer electrolytemembrane located in the stack 21 is not held in the excessively drystate and remains in the wet state, operation goes to step S100.

[0059] In the execution of step S100, both the shut-off valves 37 and 41located between the humidifier 35 and the stack 21 are applied with“open” control signals, allowing fuel gas and air to be supplied to thestack 21. Concurrently, the controller 61 applies the pump controlsignal 61 a to the pump 57 to render the flow rate thereof to have anormal value, allowing the stack 21 to begin to produce electric poweroutput.

[0060] In the succeeding step S110, the controller 61 reads out thedetected value of the temperature sensor 28 to discriminate whether thestack temperature T exceeds or is below a preset temperature Td. Whenthe stack temperature is below the preset temperature Td, operationreturns to step S120 wherein, in the same manner as in the step S80, theamount of electric power to be extracted from the stack 21 is limitedbelow the predetermined value Ph.

[0061] In the execution of the step S110, if it is found that the stacktemperature exceeds the preset temperature Td, operation then returns tostep S90 to allow the stack to operate in the normal power generationmode.

[0062] According to the first preferred embodiment of the presentinvention, the wet state of the polymer electrolyte membrane located inthe stack is detected and, even when the polymer electrolyte membraneremains in an excessively dry state, the stack can be started up afterthe wet state is recovered, enabling electric power output to beextracted from the stack in a stable fashion while avoiding the rapidvoltage drop caused during large electric power generation to enable thefuel cell system to start-up in a smooth fashion at all times.

[0063] Another advantage of the first preferred embodiment resides inthat the fuel cell system can be realized by merely incorporating asimplified structure therein to allow a displacement sensor to bemounted in the stack. As a result, further, it is possible to realizethe fuel cell system by merely adding this start-up process thereto in arelatively easy fashion.

[0064] In addition, the wet state of the stack can be detected, with aresultant decrease in trouble shooting time required for some troublesoccurred in the stack.

[0065] (Second Embodiment)

[0066]FIG. 6 is a block diagram for illustrating the structure of astarter device 71 of a fuel cell system of a second preferred embodimentaccording to the present invention. Also, the second preferredembodiment has the same basic structure as that of the fuel cell systemof the first preferred embodiment shown in FIG. 4, with like partsbearing like reference numerals as those used in FIG. 4 while detaileddescription of the like parts are omitted for the sake of clarity.

[0067] An essential feature of the second preferred embodiment differsfrom that of the first preferred embodiment in that a humidifier 73incorporates therein a heater 75.

[0068] The operation of the starter device 71 of the fuel cell system ofthe second preferred embodiment will be described in detail withreference to a general flow diagram shown in FIG. 7. The basic sequenceof operations illustrated in the general flow diagram of FIG. 7 isstored as the control program in the internal ROM of the controller 61.Also, the second preferred embodiment has the same basic steps as thosein the flow diagram of the first preferred embodiment shown in FIG. 5,with like steps bearing like reference numerals as those used in FIG. 5while a detailed description of the like steps is omitted for the sakeof clarity.

[0069] In the execution of step S130, with which the step S30 in thefirst preferred embodiment is replaced, the shut-off valve 37 locatedbetween the humidifier 73 and the stack 21 is applied with a “close”control signal to interrupt the supply of fuel gas to the stack 21while, concurrently, applying an “open” control signal to the shut-offvalve 41 located between the humidifier 73 and the stack 21 to allow airto be supplied to the stack 21. Simultaneously, the heater 75 located inthe humidifier 73 is applied with electric power 61 c, and the pumpcontrol signal 61 a is applied to the pump 57 from the controller 61 soas to maximize the rotational speed of the pump 57 for increasing theflow rate of pure water 59 to be circulated to the humidifier 73 fromthe pure water tank 55. At the same time, the timer in the controller 61is caused to begin counting operation of the incremental time.

[0070] As a result, even when the polymer electrolyte membrane locatedin the stack 21 remains in an excessively dry state at the start ofoperation, the heater 75 in the humidifier 73 is heated to produce afurther excessively humidified air, which is supplied through theshut-off valve 41 to the stack 21, with a resultant decrease in theoperating time required for wetting the polymer electrolyte membrane.

[0071] In the execution of step S150, with which the step S50 in thefirst preferred embodiment is replaced, the shutoff valve 37 locatedbetween the humidifier 35 and the stack 21 is applied with an “open”control signal to begin the supply of fuel gas to the stack 21 inaddition to supplying the air, while the heater 75 located in thehumidifier 73 is applied with electric power 61 c and applying the pumpcontrol signal 61 a to the pump 57 such that the flow rate thereof isreturned to its normal value to allow the stack 21 to begin powergeneration.

[0072] As a result, even when the polymer electrolyte membrane locatedin the stack 21 is once humidified during a predetermined time period,the heater 75 in the humidifier 73 is heated to produce a furtherexcessively humidified air, which is supplied through the shut-off valve41 to the stack 21, with a resultant decrease in the operating timerequired for wetting the polymer electrolyte membrane.

[0073] In the execution of step S170, with which the step S110 in thefirst preferred embodiment is replaced, the shut-off valves 37 and 41located between the humidifier 73 and the stack 21 are applied with“open” control signals to begin the supply of fuel gas and air to thestack 21 while applying electric power to the heater 75 located in thehumidifier 73. Simultaneously, the pump control signal 61 a is deliveredfrom the controller 61 to the pump 57 so as to regulate the flow rate toa normal value and to allow the stack 21 to begin electric powergeneration.

[0074] As a result, even when the polymer electrolyte membrane locatedin the stack 21 remains in a normal, wet state at the start ofoperation, the heater 75 located in the humidifier 73 is heated toprovide an excessively humidified air to the stack 21 via the shut-offvalve 41, allowing the polymer electrolyte membrane to be wet in areduced, operating time period.

[0075] A typical advantage of the second preferred embodiment resides inthat an increase in the amount of humidification reduces the wait timefor wetting a polymer electrolyte membrane.

[0076] In the present invention, the reliability in operation of thefuel cell system is greatly enhanced to provide a stable operation inthe fuel cell system at all times even when it remains in an excessivelydry state. Unlike a structure of a fuel cell system employing ahumidification process which begins to humidify the polymer electrolytemembrane in a normal mode at the start-up operation, the system of thepresent invention has a controller which allows the polymer electrolytemembrane of the stack to be suitably wet in the shortest period at thestart-up operation even when the polymer electrolyte membrane remains inthe excessively dry state. The obvious result is the elimination of waittime during start-up of the system and system failures that wouldotherwise occur when large power output is consumed when the stackremains in the dry state.

[0077] Since the dry state of the stack is discriminated in terms of thedisplacement value in length of the stack in the stacked direction andthe temperature of the stack, an optimum start-up control of the fuelcell system is provided with greatly simplified operating process andrapid start-up of the fuel cell system at full load can be achieved in ahighly reliable manner. This optimum start up control can be achievedwith the use of minimum number of component parts without causing aremarkable increase in cost. Thus, the system of the invention can beeasily applied to existing fuel cell systems to improve the operatingperformance.

[0078] The controller controls the fuel cell system such that, when thestack remains in the dry state, humidified air is merely supplied to thestack to render the polymer electrolyte membrane to be wet in the shortperiod. This control is achieved in an easy fashion by opening orclosing plural shut-off valves located between the humidifier and thestack.

[0079] Further, during start-up of the fuel cell system, when thepolymer electrolyte membrane of the stack remains in the dry state, thecontroller controls the humidifier such that humidified air is firstsupplied to the stack for a predetermined time period and subsequentlyhumidified fuel gas is also supplied to the stack and also controls thestack such that electric power output generated by the stack is limitedbelow a predetermined value until the polymer electrolyte membrane isbrought into a sufficiently wet state. This results in the eliminationof the voltage drops or system failures caused thereby.

[0080] Still further, a heater may be incorporated in the humidifier toprovide further excessively humidified fuel gas and air to the stack,reducing the operating time required for wetting the polymer electrolytemembrane to a sufficient level for thereby enabling the fuel cell systemto start up in a short period even when the polymer electrolyte membraneremains in the dry state.

[0081] Industrial Applicability

[0082] As described above, in the present invention, a dry state of astack in a fuel cell system is watched in terms of a displacement valuein length of the stack and a temperature of the stack, and the amount ofhumidification in fuel gas and air is controlled to allow a polymerelectrolyte membrane of the stack to be suitably wet in a highlyefficient and reliable manner. Therefore, a wide applicability thereofincluding a fuel cell system for a vehicle is expected.

1. A fuel cell system comprising: a humidifier humidifying fuel gas andair; a stack producing electric power by reacting the fuel gashumidified by the humidifier and the air humidified by the humidifier,including a plurality of fuel cells and fixed for free movement in astacked direction thereof, each of the plurality of fuel cells having apolymer electrolyte membrane; a displacement sensor detecting adisplacement value in length of the stack in the stacked direction; atemperature sensor detecting a temperature of the stack; and acontroller discriminating whether the polymer electrolyte membrane is ina dry state in response to the displacement value of the stack detectedby the displacement sensor and the temperature of the stack detected bythe temperature sensor, and controlling the humidifier, when the polymerelectrolyte membrane is discriminated as in a dry state at a start ofoperation of the stack, to cause the polymer electrolyte membrane to bebrought into a wet state.
 2. A fuel cell system according to claim 1,wherein the controller controls the humidifier, when the polymerelectrolyte membrane is discriminated as in the dry state at the startof operation, to supply the air through the humidifier to the stack fora predetermined time period while not supplying the fuel gas through thehumidifier to the stack.
 3. A fuel cell system according to claim 2,wherein the controller controls the humidifier, after only the air hasbeen supplied to the stack through the humidifier for the predeterminedtime period, to supply the fuel gas and the air through the humidifierto the stack, while controlling the stack such that the electric poweroutput from the stack is regulated to a limited value below apredetermined value until the polymer electrolyte membrane is determinedas in a wet state.
 4. A fuel cell system according to claim 1, whereinthe humidifier includes a heater heating the fuel gas and the air, andthe stack produces the electric power by reacting the fuel gashumidified and heated by the humidifier and the air humidified andheated by the humidifier.
 5. A fuel cell system according to claim 4,wherein the controller controls the humidifier, when the polymerelectrolyte membrane is discriminated as in the dry state at the startof operation, to supply the air through the humidifier to the stack fora predetermined time period while not supplying the fuel gas through thehumidifier to the stack.
 6. A fuel cell system according to claim 5,wherein the controller controls the humidifier, after only the air hasbeen supplied to the stack through the humidifier for the predeterminedtime period, to supply the fuel gas and the air through the humidifierto the stack, while controlling the stack such that the electric poweroutput from the stack is regulated to a limited value below apredetermined value until the polymer electrolyte membrane is determinedas in a wet state.
 7. A fuel cell system according to claim 1, whereinthe controller controls the humidifier to supply the fuel gas and theair through the humidifier to the stack when the polymer electrolytemembrane is discriminated as in a wet state at the start of operation ofthe stack.
 8. A fuel cell system according to claim 7, wherein thecontroller controls the humidifier to supply the fuel gas and the airthrough the humidifier to the stack, while controlling the stack suchthat the electric power output from the stack is regulated to a limitedvalue below a predetermined value until the temperature of the stackexceeds a predetermined temperature.
 9. A fuel cell system comprising:humidifying means humidifying fuel gas and air; a stack producingelectric power by reacting the fuel gas humidified by the humidifyingmeans and the air humidified by the humidifying means, including aplurality of fuel cells and fixed for free movement in a stackeddirection thereof, each of the plurality of fuel cells having a polymerelectrolyte membrane; a displacement detecting means detecting adisplacement value in length of the stack in the stacked direction;temperature detecting means detecting a temperature of the stack; andcontrolling means discriminating whether the polymer electrolytemembrane is in a dry state in response to the displacement value of thestack detected by the displacement detecting means and the temperatureof the stack detected by the temperature detecting means, andcontrolling the humidifying means, when the polymer electrolyte membraneis discriminated as in a dry state at a start of operation of the stack,to cause the polymer electrolyte membrane to be brought into a wetstate.
 10. A method of controlling a fuel cell system provided with ahumidifier humidifying fuel gas and air, and a stack producing electricpower by reacting the fuel gas humidified by the humidifier and the airhumidified by the humidifier, including a plurality of fuel cells andfixed for free movement in a stacked direction thereof, each of theplurality of fuel cells having a polymer electrolyte membrane, themethod comprising: detecting a displacement value in length of the stackin the stacked direction; detecting a temperature of the stack;discriminating whether the polymer electrolyte membrane is in a drystate in response to the displacement value of the stack detected by thedisplacement sensor and the temperature of the stack detected by thetemperature sensor; and controlling the humidifier, when the polymerelectrolyte membrane is discriminated as in a dry state at a start ofoperation of the stack, to cause the polymer electrolyte membrane to bebrought into a wet state.